4 Answers
4

Fiddler is technically a web debugging tool, but it does so by acting as a proxy. It comes with a built-in scripting environment. It would be trivial to implement some arbitrary "proxy rules" in a small script.

Just download it and follow the installation instructions on the website. You shouldn't have to do anything special to configure it for your purpose, just start it and point your software to the server's IP address at port 3128.

how can I 'disable' out-bound access to the internet in order to simulate a real proxy enviornment?
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Craig JohnstonMar 25 '11 at 1:51

You can't fully disable the Internet on a Windows XP machine without also disabling it for the proxy server. If you have third-party firewall software, you might be able to use that to prevent your program from accessing the Internet. (The built-in firewall in Windows XP does not support outbound connection blocking.) Otherwise you'd need two computers or a virtual machine to create an environment with one computer networked with no Internet access for the program and the other networked to the former with Internet access running the proxy server.
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PatchesMar 25 '11 at 1:55

I have another machine on the network which I could use. How do I prevent it accessing the internet other than via the proxy server?
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Craig JohnstonMar 25 '11 at 1:58

@Craig: Assuming a wired Ethernet connection: running netsh interface ip "Local Area Connection" gateway=none on a Command Prompt should do the trick, but depending on your network configuration that might cut you off from the network too. If your computers IP addresses are statically assigned you'll need to change the default gateway back to it's original value when you're done. If it's assigned via DHCP just run ipconfig /release and then ipconfig /renew to set everything back to normal.
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PatchesMar 25 '11 at 2:09

I can recommend Proxy+. It is free for 3 users, contains SOCKS-proxy, runs as a service and managed via Web-browser.

If you want to send all traffic through proxy set proxy server in Internet Explorer settings and type proxycfg -p 127.0.0.1:3128 in command prompt. This should work for most of software. For those apps that does not respect system settings you can use something like Free Cap.

To block network traffic for applications individually you can use 3rd-party firewall. I prefer Outpost Firewall Free - it is free and easy to configure.